1. Field of the Invention
The present invention relates to a projection exposure apparatus for manufacturing semiconductor elements and, more particularly, to an exposure apparatus of a system for correcting and controlling changes in imaging characteristics caused by changes in optical characteristics of exposure energy incident on a projection optical system.
2. Related Background Art
Matching precision is one of the most important factors in performance of a reduction or equal-size type projection exposure apparatus
A factor greatly influencing the matching precision is magnification error of a projection optical system. The sizes of circuit patterns used for VLSIs have been decreased year by year, and line widths of micropatterns have also been subjected to miniaturization. Demand has arisen for improving matching precision. In order to satisfy such demand, it is necessary to keep projection magnification at a predetermined value. The magnification of a projection optical system is strictly adjusted when an apparatus incorporating such a system is manufactured and is installed in a production line. However, it is known that magnifications vary with respect to a predetermined magnification due to slight changes in temperature of the apparatus, slight barometric and temperature changes in a clean room, or emission of light incident on the projection optical system during the exposure. Under these circumstances, various conventional methods are proposed wherein errors caused by changes in magnification of the projection optical system are automatically corrected to maintain a predetermined magnification on a workpiece such as a wafer. Examples are a method of changing a distance between a reticle (or a mask) and a projection lens along the axial direction, a method of moving a specific element in a projection lens, and a method of controlling an air pressure in a specific air chamber in a projection lens to change a refractive index in this air chamber.
Among these conventional control methods, the last method has an advantage in that mechanically movable members are not used and a change in refractive index of air is utilized, thereby achieving magnification control with very high precision. Magnification control by such barometric control is disclosed in U.S. Ser. No. 632,335 filed on July 19, 1984, and the corresponding Japanese application.
According to a system described in this prior patent, a projection image (e.g., 15 mm square) of a pattern on a wafer can be fine-adjusted in the range of 0.02 .mu.m.+-.0.5 .mu.m. So-called focal point changes also occur wherein a focal point (i.e., the position of the imaging plane) of the projection lens is changed along the axial direction. The focal point changes can also be corrected by barometric control.
Magnification and focal point changes caused by light incident on the projection lens, i.e., light passing through the reticle can be defined as storage of heat in the projection lens. In a normal projection lens, the wavelength components of the illumination light can be transmitted with high efficiency. However, some of the components are absorbed by the lens element or the like and are converted into heat.
Since heat storage of the projection lens is a heat diffusion phenomenon having a given time constant, the influences acting on optical characteristics such as the magnification and the focal point are the sum of influences of the previous incident light. By utilizing a given method, e.g., opening/closing operation of a shutter for emitting or shielding light to or from the reticle, history information of the incident light is produced, and barometric control is performed on the basis of the history information. In this manner, changes in optical characteristics caused by light incident on the projection lens during exposure can be sequentially corrected. Such an apparatus is described in detail in U.S. Ser. No. 656,777 filed on Oct. 1, 1984, now U.S. Pat. No. 4,666,273, issued May 19, 1987, and the corresponding Japanese application. According to this control method, history information of light incident on the projection lens is the most important factor. The history information is defined as data for solely specifying the changes in optical characteristics of the projection lens at present on the basis of the incident state of the previous light, and the like.
Since the history information represents a sum of influences of all previous light components, its loss for some reason (e.g., a power failure) during operation undesirably disables normal correction control of magnification. In addition, if errors occur in the history information, accurate control cannot be expected. In order to prevent this, a method is proposed wherein exposure is disabled from the time when the history information is lost or errors occur therein, for a short period of time for which influences of the history of the incident light can be neglected. After the projection lens is cooled, exposure under correction control in a state free from the influences of the history is restarted. However, it takes a long period of time to restore normal exposure processing. This control method is not suitable for high productivity. For example, when this method was used in a given test apparatus, it took 90 minutes or more (i.e., the apparatus down time) to cool the apparatus if safety time is included.